Disposition of [1′-14C]Stavudine after Oral Administration to Humans

The disposition of stavudine, a potent and orally active nucleoside reverse transcriptase inhibitor, was investigated in six healthy human subjects. Before dosing humans with [1'-C-14] stavudine, a tissue distribution study was performed in Long-Evans rats. Results from this study showed no accumulation of radioactivity in any of the tissues studied, indicating that the position of the C-14-label on the molecule was appropriate for the human study. After a single 80-mg (100 mu Ci) oral dose of [1'-C-14] stavudine, approximately 95% of the radioactive dose was excreted in urine with an elimination half-life of 2.35 h. Fecal excretion was limited, accounting for only 3% of the dose. Unchanged stavudine was the major drug-related component in plasma (61% of area under the plasma concentration-time curve from time zero extrapolated to infinite time of the total plasma radioactivity) and urine (67% of dose). The remaining radioactivity was associated with minor metabolites, including mono-and bis-oxidized stavudine, glucuronide conjugates of stavudine and its oxidized metabolite, and an N-acetylcysteine (NAC) conjugate of the ribose (M4) after glycosidic cleavage. Formation of metabolite M4 was shown in human liver microsomes incubated with 2',3'-didehydrodideoxyribose, the sugar base of stavudine, in the presence of NAC. In addition, after similar microsomal incubations fortified with GSH, two GSH conjugates, 3'-GS-deoxyribose and 1'-keto-2',3'-dideoxy-3'-GS-ribose, were observed. This suggests that 2',3;-didehydrodideoxyribose underwent cytochrome P450-mediated oxidation leading to an epoxide intermediate, 2',3'-ribose epoxide, followed by GSH addition. In conclusion, absorption and elimination of stavudine were rapid and complete after oral dosing, with urinary excretion of unchanged drug as the predominant route of elimination in humans.